Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer.
Due to the fact that the thin support softens when heated during the printing operation and then sticks to the thermal printing head thereby causing malfunctioning of the printing apparatus and reduction in image quality the backside of the support (side opposite to the dye layer) is typically provided with a slipping layer to facilitate passage of the dye-donor element under the thermal printing head. An adhesive layer may be provided between the support and the slipping layer.
The slipping layer generally comprises a lubricating material and a binder. In the conventional slipping layers the binder is either a cured binder (radiation- or heat-cured) or a polymeric thermoplast.
Using polymeric thermoplasts as binder for the slipping layer such as i.a. poly(styrene-co-acrylonitrile), polystyrene and polymethylmethacrylate has the disadvantage of relatively low heat stability of the slipping layer containing said binder en therefore unsatisfactory performance of said slipping layer. Further when dye-donor elements having such slipping layers have been rolled up and stored for any length of time such that the backcoat of one portion of the donor element is held against the dyecoat of another portion, sticking of the backcoat to the dyecoat occurs and migration of the dye takes place leading to a loss of density of any prints eventually made using that donor element.
A disadvantage of using radiation-cured binders for the slipping layer (as described in, for example, EP 329117, JP 60/151096, JP 60/229787, JP 60/229792, JP 60/229795, JP 62/212192 and JP 02/128899) is their cumbersome manufacture.
The actually used cross-linking agent in the heat-curable binder systems for the slipping layer (as described in, for example, EP 153880, EP 194106, EP 324946, JP 62/227787, JP 62/259889, JP 63/51189, JP 01/5884 and JP 01/51980) is a polyisocyanate, which is highly toxic and therefore is to be avoided. A further problem encountered upon using polyisocyanate heat-curable binder systems is the limited pot life of the binder composition.